Problem: Flash plugin crashes in Firefox on 64 bit Debian lenny whenever entering fullscreen mode. The console displays an error message like "libflashplayer.so: undefined symbol: gtk_widget_get_window". Both Firefox and Flash plugin were installed manually rather than from a Debian package on the affected system.
Solution: This problem occurs due to an outdated version of 32 bit libgtk2 in Debian lenny (ia32-libs-gtk), which doesn't have the required symbol. As a remedy, you should install 64 bit versions of libgtk2, Firefox and Flash player from Debian packages. However, these packages are not found in lenny repository, so you first need to add the backports repository to your apt (important: also add
main in the repository definition!) After that, just go ahead with
apt-get -t lenny-backports install flashplugin-nonfree iceweasel libgtk2.0-0